Design Considerations for Environmental

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ACI 350.4R-04 became effective February 27, 2004.Copyright 2004, American Concrete Institute.All rights reserved including rights of reproduction and use in any form or by any

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Reference to this document shall not be made in contractdocuments. If items found in this document are desired by theArchitect/Engineer to be a part of the contract documents, theyshall be restated in mandatory language for incorporation bythe Architect/Engineer.

350.4R-1

It is the responsibility of the user of this document toestablish health and safety practices appropriate to the specificcircumstances involved with its use. ACI does not make anyrepresentations with regard to health and safety issues and theuse of this document. The user must determine theapplicability of all regulatory limitations before applying thedocument and must comply with all applicable laws andregulations, including but not limited to, United StatesOccupational Safety and Health Administration (OSHA)health and safety standards.

Design Considerations for Environmental

Engineering Concrete Structures

ACI 350.4R-04

Environmental engineering concrete structures provide conveyance, storage,

and treatment of water, wastewater, and other materials. This report outlines

special design considerations such as loads, stability, joint details, and

special design conditions that are unique to these types of structures as

well as ancillary structures.

Keywords: buoyancy; clarifier; contraction; design; expansion; filler;

flood; flotation; forces; hazardous; ice; impact; joint; load; overturning;

reservoir; safety; sealant; sliding; stability; tank; tension; torque; vibration;

waterstop; weights.

TABLE OF CONTENTSChapter 1General, p. 350.4R-2

1.1Scope

1.2Related documents

Chapter 2Design loads, p. 350.4R-22.1Floor live loads

2.2Contained fluid and sludge loads

2.3External earth loads

2.4External fluid loads

2.5Environmental loads

2.6Other design loads

Chapter 3Stability considerations, p. 350.4R-6

3.1Flood considerations3.2Sliding and overturning considerations

Chapter 4Special design conditions, p. 350.4R-94.1Load combinations

4.2Expansion and contraction conditions

4.3Foundation conditions

Reported by ACI Committee 350

James P. Archibald* William Irwin Jerry Parnes

Jon B. Ardahl Keith W. Jacobson Andrew R. Philip

John W. Baker Dov Kaminetzky Narayan M. Prachand

Walter N. Bennett M. Reza Kianoush Satish K. Sachdev

Steven R. Close David G. Kittridge William C. Schnobrich

Anthony L. Felder Dennis C. Kohl John F. Seidensticker

Carl A. Gentry Nicholas A. Legatos William C. Sherman

Clifford Gordon|| Larry G. Mrazek Lawrence J. Valentine

Paul Hedli Javeed A. Munshi Miroslav Vejvoda

Jerry A. Holland Paul Zoltanetzky, Jr.

Charles S. HanskatChair

Lawrence M. TabatSecretary

*Committee Secretary while this document was being prepared.Committee Chair while this document was being prepared.Co-chair of subcommittee that prepared this document.Members of subcommittee that prepared this document.||Deceased.

yright American Concrete Instituteded by IHS under license with ACI Licensee=/5959825001, User=alkadhimi, ahmad

Not for Resale, 11/28/2005 10:36:50 MSTeproduction or networking permitted without license from I HS

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350.4R-2 ACI COMMITTEE REPORT

4.4Design and detailing considerations

4.5Vibration conditions

4.6Hazardous design conditions

4.7Corrosive conditions

4.8Construction conditions

Chapter 5Joints in concrete, p. 350.4R-135.1General

5.2Construction joints

5.3Movement joints

5.4Waterstops

5.5Joint fillers

5.6 Joint sealants

Chapter 6References, p. 350.4R-156.1Referenced standards and reports

6.2Cited references

CHAPTER 1GENERAL1.1Scope

This report outlines design considerations that are unique

to environmental engineering concrete structures and associated

buildings. Environmental engineering concrete structures are

defined in ACI 350 as concrete structures intended for

conveying, storing, or treating water, wastewater, or other

nonhazardous liquids, and for the secondary containment of

hazardous liquids. Applicable building codes and other

industry standards should be consulted for load and design

considerations not included herein. The engineer should

check with the local building department to confirm the

applicable building code for the project location and determine

if there are any local amendments.

The structural design recommendations given herein

should be regarded as common industry practice and arerecommended for general use. Any special structural

features, unusual loading conditions, or unusual exposure

conditions may require special design considerations to

achieve a higher level of performance than implied by these

minimum recommendations.

1.2Related documentsEnvironmental engineering concrete structures should be

designed and constructed in conformance with ACI 350/

350R, 350.1, 350.2R, and 350.3. References 1 through 3 may

also be useful in the design of liquid-containing structures.

CHAPTER 2DESIGN LOADS2.1Floor live loadsFloor live loads in equipment and process areas generally take

into account fixed equipment weights, stored material loads, and

normal live loads due to personnel and transient loads. Floor live

loads should account for installation, operation, and maintenance

of equipment, and possible modifications or changes in use.

During installation or maintenance, portions of equipment

may be laid down at various locations on the floor. For example,

heavy electrical equipment may be temporarily placed near the

center span of a floor during installation or maintenance, even

though its final location may be near support locations.

Weights of concrete bases for equipment may also be

included in floor live loads, and consideration should be given

to weights of piping, valves, and other equipment accessories

that may be supported by the floor slab. Consequently,

conservative uniform live loads are recommended.

Information on estimated equipment weights and footprints

should be obtained so that design floor live loads can be verified.

The engineer may consider distribution of the equipment

loads over an area greater than the footprint dimensionsusing engineering judgment. Because actual equipment

weights from various equipment suppliers may vary,

conservative estimates of equipment weights should be

used. A minimum floor live load of 150 lb/ft2 (7.2 kPa) is

commonly used for slabs that support equipment. Heavier

live loads are common in electrical equipment rooms. Generally,

stairways and walkways should be designed for a minimum live

load of 100 lb/ft2 (4.8 kPa). Where loads on catwalks are

expected to be limited, a minimum live load of 40 lb/ft2 (1.9 kPa)

may be used in accordance with ASCE 7.

Large pieces of equipment may be assembled in their final

fixed location. While temporary laydown of individual pieces of

equipment should still be considered, it may be permissible toconsider the total weight of the equipment only in its fixed

location on the floor. Additionally, operational loads should be

considered with the equipment in its fixed location. Operational

loads may include thrusts, torques, contained fluids or sludge, or

impact. For example, supports for vertical turbine pumps should

include the weight of the vertical column of water in the riser,

and sludge press loads should include the weight of the sludge

being processed in the press.

In areas where chemicals or other materials are stored, the

maximum weight of stored material should be determined

based on the height and density or specific gravity of the

material and its container(s). The material densities listed in

Table 2.1(a) and(b)may be used for estimating applicableloads. ASCE 7 may be referenced for other common material

densities and floor live loads. Chemicals can be delivered

and stored by a variety of methods and mediums, including

bags, barrels, bottles, cylinders, drums, kegs, pails, rail cars,

sacks, totes, or trucks. The engineer should confirm the

delivery method and storage method for design.

Caution should be used in applying floor live load reductions

as permitted by building codes, due to the greater likelihood

of simultaneous distributed loading in some equipment and

chemical storage areas. Consider the potential change of use

of adjacent areas when setting the floor live load. It is preferable

to use the same design live load in adjacent areas where

practical. Floor live loads should be posted as indicated in

the applicable building code and should be identified on the

design drawings.

2.2Contained fluid and sludge loadsThe principal applied loads on liquid-containment

structures are due to the fluid pressures on the walls and

slabs caused by the contained fluids. The following densities

are conservative values for calculating equivalent fluid pressures

of common environmental materials encountered that may

be used in structural design:

Raw sewage 63 lb/ft3 (1000 kg/m3)



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ENVIRONMENTAL ENGINEERING CONCRETE STRUCTURES 350.4R-3

Grit excavated from grit chamber 110 lb/ft3 (1800 kg/m3)

Digested sludge, aerobic 65 lb/ft3 (1000 kg/m3)

Digested sludge, anaerobic 70 lb/ft3 (1100 kg/m3)

Thickened or dewatered sludge 63 to 85 lb/ft3

(1000 to 1400 kg/m3)

(depending on moisture content)

Fluid loads should be considered for both the normal fluid

levels and for the worst-case fluid level. One such worst-case

design condition is where the fluid is at the top of the

containment structure or at the level at which overflow

would occur elsewhere in the hydraulic system, such that

high fluid levels could not occur at the location being

evaluated. Many liquid-containment structures have

encountered such overflow conditions in the past. The

code-required load factors and environmental durability

factors apply to normal maximum fluid levels. Code-required

Table 2.1(a)Densities in inch-pound units of chemical for structural design (refer to Reference 4 forlisting of selected chemicals)

Chemical Density, lb/ft3 Chemical Density, lb/ft3

Acetic acid 65 (liquid) Fluosillicic acid 79 (liquid at 30%)

Activated carbon Powder 8 to 28; average 12 Hydrochloric acid 73 (liquid at 35%)

Activated silica Approximately 90 (liquid) Hydrofluoric acid 73 (liquid at 55%)

Alum, liquid 83 (liquid at 60 F) Hydrogen peroxide 75 (liquid at 50%)

Aluminum ammonia sulfate 70 (granular or powder) Methanol 98 (liquid)

Aluminum chloride solution 72 (liquid) Oxygen 71 (liquid)

Aluminum potassium sulfate 70 (granular or powder) Phosphoric acid 98 (liquid at 75%)

Aluminum sulfate60 to 75 (granular,

powder); 84 (liquid at 50%) Polyaluminum chloride 91 (liquid at 5%)

Ammonia, anhydrous 43 (liquid at 28 F) Polyelectrolyte or polymer Dry 88; liquid 62 to 92

Ammonia, aqua (ammonium hydroxide) 56 (liquid at 60 F) Polyphosphate (zinc orthophosphate) 80 to 100 (liquid)

Ammonia silicoflouride 80 (crystals) Potassium aluminum sulfate 67 (crystals)

Ammonium aluminum sulfate(ammonium alum) 75 (crystals) Potassium permanganate

102 (powder);64 (3% solution)

Ammonium sulfate 60 (damp); 49 to 64 (dry) (crystals) Sodium aluminateHigh-purity 50; standard

60 (powder, crystals);98 (45% solution)

Barium carbonate 52 to 78 (powder) Sodium bicarbonate 62 (granular, powder)

Bentonite Powder 45 to 60;granules 75 Sodium bisulfate 70 to 85 (powder, crystals)

Bromine 194 (liquid) Sodium carbonate (soda ash)Dense 65;

medium 40; light 30(granular, powder)

Calcium carbonate Powder 35 to 60;granules 115 Sodium chloride Rock 60; crystal 78;powder 66

Calcium hydroxide (hydrated lime) 20 to 50 (powder) Sodium chlorite 80 (25% solution)

Calcium hypochlorite Granules 80; powder 32 to 52 Sodium fluoride Powder 65 to 100;granules crystal 106

Calcium oxide (quick lime, pebble lime) 55 to 70; 60 typicalhopper load (pebbles) Sodium fluorosilicate 72 (powder)

Carbonic acid (carbon dioxide solution) 62 (liquid) Sodium hexametaphosphate(sodium polysulfate)Glass 64 to 100; powder and

granular 44 to 60

Chlorinated lime 50 (powder) Sodium hydroxide Pellets 70; flakes 46 to 62; 95(50% solution)

Chlorine 92 (liquid) Sodium hypochlorite 76 (liquid at 15%)

Citric acid 77 (liquid at 50%) Sodium silicate 88 (liquid)

Copper sulfate Crystal 90; powder 68 Sodium silicoflouride Granular 85 to 105;powder-granular 60 to 96

Diatomaceous earthNatural 5 to 18; calcined 6 to 13;

flux-calcined 10 to 25(fibrous material)

Sodium sulfate 70 to 100(crystals, powder)

Disodium phosphate Crystal hydrate 90;anhydrous 64 Sodium sulfitePowder 80; granular 107;

liquid 82 (at 12.5%)

Dolomitic hydrated lime 30 to 50 (powder) Sodium thiosulfate 60 (granules, crystals)

Dolomitic limePebble 65;

ground or lump 50 to 65;powder 37 to 65; average 60

Sulfur dioxide 89.6 at 32 F (liquid)

Ferric chloride 93 (liquid); crystal 64;anhydrous 45 to 60 Sulfuric acid 115 (liquid)

Ferric sulfate 72 (granular) Tetrasodium pyrophosphate Crystal 50 to 70;powder 46 to 68

Ferrous chloride 86 (liquid at 35%) Trisodium phosphate Crystal 60; monohydrate 65;anhydrous 70

Ferrous sulfate 66 (granular, powder)



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factors intended to improve durability may not be applicable

to worst-case load conditions.

For enclosed liquid-containment structures, consideration

should also be given to internal positive or negative air pressures

caused by rapid filling or emptying of the containment structure.

Positive and negative air pressures can also be caused by

induced ventilation, such as for odor control. The worst-case

design for negative pressure may be due to pipe rupture and

rapid drawdown of the tank contents, and the maximum positive

pressure is related to the maximum fill rate of the equipment.

Generally, suitably sized gooseneck vents should be

provided at top slabs to alleviate significant variations in

Table 2.1(b)Densities in metric units of chemicals for structural design (refer to Reference 4 for listing ofselected chemicals)

Chemical Density, kg/m3 Chemical Density, kg/m3

Acetic acid 1000 (liquid) Fluosillicic acid 1300 (liquid at 30%)

Activated carbon Powder 100 to 450;average 190 Hydrochloric acid 1200 (liquid at 35%)

Activated silica Approximately 1400 (liquid) Hydrofluoric acid 1200 (liquid at 55%)

Alum, liquid 1300 (liquid at 16 C) Hydrogen peroxide 1200 (liquid at 50%)

Aluminum ammonia sulfate 1100 (granular or powder) Methanol 1600 (liquid)

Aluminum chloride solution 1200 (liquid) Oxygen 1100 (liquid)

Aluminum potassium sulfate 1100 (granular or powder) Phosphoric acid 1600 (liquid at 75%)

Aluminum sulfate 960 to 1200 (granular,powder); 1300 (liquid at 50%) Polyaluminum chloride 1500 (liquid at 51%)

Ammonia, anhydrous 690 (liquid at 33 C) Polyelectrolyte or polymer Dry 1400; liquid 990 to 1500

Ammonia, aqua (ammonium hydroxide) 900 (liquid at 16 C) Polyphosphate(zinc orthophosphate) 1300 to 1600 (liquid)

Ammonia silicoflouride 1300 (crystals) Potassium aluminum sulfate 1100 (crystals)

Ammonium aluminum sulfate(ammonium alum) 1200 (crystals) Potassium permanganate

1600 (powder);1000 (3% solution)

Ammonium sulfate 960 (damp);780 to 1000 (dry) (crystals) Sodium aluminateHigh-purity 800; standard 960

(powder, crystals);1570 (45% solution)

Barium carbonate 830 to 1200 (powder) Sodium bicarbonate 990 (granular, powder)

Bentonite Powder 720 to 960; granules 1200 Sodium bisulfate 1100 to 1400(powder, crystals)

Bromine 3100 (liquid) Sodium carbonate (soda ash)Dense 1000;

medium 640; light 480(granular, powder)

Calcium carbonate Powder 560 to 960; granules 1800 Sodium chloride Rock 960; crystal 1200;powder 1100

Calcium hydroxide (hydrated lime) 320 to 800 (powder) Sodium chlorite 1280 (25% solution)

Calcium hypochlorite Granules 1300; powder 510to 830 (pebbles) Sodium fluoridePowder 1100 to 1600;granules crystal 1700

Calcium oxide (quick lime, pebble lime) 880 to 1100; 960 typicalhopper load (pebbles) Sodium fluorosilicate 1200 (powder)

Carbonic acid(carbon dioxide solution) 990 (liquid)

Sodium hexametaphosphate(sodium polysulfate)

Glass 1000 to 1600;powder and granular

700 to 960

Chlorinated lime 800 (powder) Sodium hydroxide Pellets 1100; flakes 740 to 990;1520 (50% solution)

Chlorine 1500 (liquid) Sodium hypochlorite 1200 (liquid at 15%)

Citric acid 1200 (liquid at 50%) Sodium silicate 1400 (liquid)

Copper sulfate Crystal 1400; powder 1100 Sodium silicoflourideGranular 1400 to 1700;

powder-granular960 to 1500

Diatomaceous earth Natural 80 to 290; calcined 100 to210; flux-calcined 160 to 400(fibrous material)

Sodium sulfate 1100 to 1600 (crystals, powder)

Disodium phosphate Crystal hydrate 1400;anhydrous 1000 Sodium sulfitePowder 1300; granular 1700;

liquid 1300 (at 12.5%)

Dolomitic hydrated lime 480 to 800 (powder) Sodium thiosulfate 960 (granules, crystals)

Dolomitic limePebble 1000; ground or lump 800

to 1000; powder 590 to 1000;average 960

Sulfur dioxide 1400 at 0 C (liquid)

Ferric chloride 1500 (liquid); crystal 1000;anhydrous 720 to 960 Sulfuric acid 1800 (liquid)

Ferric sulfate 1200 (granular) Tetrasodium pyrophosphate Crystal 480 to 1100;powder 740 to 1100

Ferrous chloride 1400 (liquid at 35%) Trisodium phosphate Crystal 960; monohydrate 1000;anhydrous 1100

Ferrous sulfate 1100 (granular, powder)



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internal air pressures. Where pressure or vacuum relief

valves are provided to limit such pressures, structural

design should account for minimum and maximum

settings of such valves. Vents and relief valves should be

designed such that they do not plug, freeze, or become

inoperable due to corrosion.

At filter basins, walls may be subject to full hydrostatic

pressure during backwashing, even though pressures may beless during the normal filtering mode. Where internal walls

of liquid-containment structures will not be exposed to

unbalanced static fluid loads due to flow arrangement,

such walls should be designed for a minimum 6 in. head

differential unless hydraulic analysis indicates a higher

differential. Larger differential heads might occur during

filling or draining operations than during normal flow

conditions. Trashracks (bar racks) and other screening

devices should be designed considering differential water

pressure due to clogging. Such screening devices are

often designed assuming a fully clogged condition with

full hydrostatic head.

2.3External earth loadsWalls subject to earth pressure should be carefully evaluated

for structure-soil interaction. This evaluation should include

determinations as to whether the wall is able to deflect

sufficiently to reduce lateral earth pressures from at-rest soil

pressures to active soil pressures and whether pressure diagrams

due to backfilling are more trapezoidal than triangular.

Because of conservative criteria used for design of liquid-

containment structures, such walls are typically stiffer than

common retaining walls. At-rest earth pressures should be

assumed unless calculated deflections justify lower lateral

earth pressures. Equivalent fluid pressures representing

at-rest pressures resulting from equivalent fluid densityvalues of 60 lb/ft3 (960 kg/m3) above the ground water table

and 95 lb/ft3 (1500 kg/m3) below the ground water table are

commonly assumed for preliminary design, but should be

confirmed for final design.

Fully developed passive earth pressures may be associated

with relatively large movements and, therefore, should be

used with caution. The deformation required to mobilize 1/2

of the passive pressure is significantly smaller than that

required for full mobilization.5 Where the soil can be relied

on to resist lateral loads and the resulting movement can be

tolerated, passive pressures may be used. If passive pressure is

used to resist lateral loads, pipes and other utilities connected to

the structure should be designed for the expected movement.

Design pressures on the structure should be based on the

passive pressures developed to resist the lateral loads. For

overturning and bearing-capacity analyses, resisting pressures

used in design should not exceed 1/2 the maximum passive

pressure.5 Where the effects of lateral soil resistance are

included in the design of retaining walls for some loading

conditions, consideration should also be given to construction

loading conditions. For construction loading conditions, the

resisting soil may not be in place.

Where structures are buried, the roof loads should include

the weight of the earth cover and applied loading due to vehicles

or equipment used for placing the earth fill or accessing the

interior of the structure. Pattern loading effects can be

critical for buried roofs, and consideration should be

given to some spans subjected to full earth and equipment

loads with adjacent spans unloaded. These effects are

particularly significant in flat slab roof systems and

similar continuous span systems. Construction documents

should indicate any restrictions on the placement, type, or

weight of equipment to be used, and on the sequence or

lift thicknesses for the earth cover. For the completed

structure, surface live loads should be included to account

for the proposed use. Loading limits should be posted to

avoid overloading the structure during operation.

Where vehicles have access adjacent to walls, a surcharge

equal to 2 ft (600 mm) of soil is commonly used in accordance

with AASHTO Standard Specifications for Highway Bridges.

2.4External fluid loadsExternal fluid pressures should be considered in addition

to external earth pressures. Hydrostatic pressures outside

of structures may occur due to high ground water conditionsand floods. Elevated ground water conditions may occur

due to leakage from adjacent liquid-containing structures

and pipes or due to inadequate drainage around a structure.

External fluid loads increase the effective lateral loads on

the walls and may also cause flotation of the structure. All

liquid-containing structures or individual cells of structures

should be evaluated for the empty condition. Refer to

Section 3.1 for additional discussion relating to floods

and flotation.

2.5Environmental loadsEnvironmental structures should also be designed for

common environmental design loads such as wind, snow,thermal effects, and earthquakes. Such loads are typically

defined in building codes and other industry standards. An

appropriate importance factor should be selected from the

building code unless a higher factor is requested by the client

for improved reliability. Ice loads may also be significant for

some structures.6 Analysis and design should be in conformance

with the ACI 350 code.

Seismic design for liquid-containment structures should

be in accordance with ACI 350.3 where applicable. Interior

walls and baffle walls that are designed for minimum static

pressure should be designed for differential pressures due to

fluid sloshing loads. Effects of lateral loads on equipment

design and on equipment anchorages, including dynamic

fluid loads, should also be considered. Fluid sloshing may

induce additional loads on equipment appurtenances, such as

for clarifiers and for paddle mixers. The center column

supports for clarifiers should be designed to accommodate

such additional forces.

Lateral earth pressures may be increased or decreased due

to horizontal ground acceleration caused by earthquakes.

The Mononobe-Okabe pseudostatic approach is commonly

used to evaluate such load effects.7-9 The seismic component

of the lateral soil pressure is commonly assumed to be

located at the upper third point of the soil height.5 Vertical



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accelerations due to earthquakes may also increase or decrease

effective fluid pressures and effective weights of materials.

2.6Other design loads2.6.1 Impact loadsImpact loads should be included

where appropriate, such as from cranes and monorails, vehicles,

elevators, lifting hooks, transient pressures in pipes, and due to

equipment operation. An appropriate impact allowance as apercentage of the equipment weight may be used in accordance

with recommendations of standard codes and specifications.

Alternatively, a detailed transient load, dynamic analysis

may be performed.

2.6.2Loads at pipe penetrationsPressure thrust loads at

pipe penetrations through walls should be considered in wall

design. The transfer of thrust from the pipe to the wall

depends on restraint of the pipe within the wall and on

restraint of the pipe joints on each side of the wall penetration. A

fully restrained pipe may be able to carry thrust forces

internally within the pipe material without transmitting

external thrust to the wall of the structure. Each case

should be examined for balancing the internal pipe thrustforces at bends and at joints. The design thrust force

should be based on maximum test pressure or on transient

pressure conditions, as applicable.

Horizontal pipe sections on the exterior side of walls may

induce loads into the wall due to settlement of soil below the pipe

section. Design and detailing should account for such conditions.

2.6.3Forces at gatesAt sluice gates and slide gates, gate

hoists may induce significant loads into the structure, including

eccentric loading on walls where brackets or corbels are

provided for support of floor stands. Gates are often

forced open or closed when sticking occurs or when

obstructions are encountered, and concrete supportmembers have often cracked due to the large upward or

downward forces that develop.

AWWA C 560 and AWWA C 513 specify required design

forces in stems for cast-iron sluice gates and fabricated-

metal slide gates. Based on these requirements, concrete

corbels and metal brackets used to support gate operators

should be designed for the following forces:

A minimum of 2-1/2 times the output thrust of the gate

operator based on a 40 lb (180 N) effort applied at the

crank for all gates with cranks; and

The output thrust of the operator based on the stalled

motor torque for electric motor operators or the cylinder

capacity at maximum working pressure for hydraulic and

pneumatic operators.

These forces may act in either direction. Both the

supporting wall and corbel or bracket need to be designed to

resist these forces.

2.6.4Forces at clarifiersMechanisms in circular clarifiers

generally are supported on a center column. This center

column should resist large torques due to sludge loads on the

long radial scrapers. As a minimum, the center column

should be designed to resist the stalling torque of the scraper

mechanism or the torque corresponding to the setting of the

mechanism limit switch. This force can be considerableas

much as 5000 ft-kip (7000 kN-m) on a center column in a

500 ft (150 m) diameter clarifier.

The center column foundation should also be designed to

resist this torque. The resisting moment arms of the soil friction

and the earth pressure on the center foundation are relatively

small, and it is possible for the center column foundation to slip

and rotate. One solution for resisting this torque is to connect the

center column foundation to the clarifier base slab. The torsionalresistance of the foundation can also be increased by the use of

batter piles at the perimeter of the foundation. Increasing the

friction by increasing the foundation weight or increasing the

area of the foundation to give a bigger lever arm, or both, will

also provide greater torsional resistance.

Refer also to Section 2.5 regarding seismic forces at clarifiers.

2.6.5 Internal pressure and vacuumSome environmental

engineering concrete structures are designed for positive or

negative gas pressures relative to atmospheric pressure. For

example, anaerobic digesters typically operate at a low gas

pressure of a few inches of water column. Air handling

equipment for odor control will typically cause a slight negative

gas pressure. Structures should be designed for the

maximum anticipated internal pressures.

For safety, pressure and vacuum relief valves, or functionally

equivalent systems, are installed on digesters to prevent the

buildup of excessive gas pressure or vacuum. The pressure and

vacuum relief valves are typically set to relieve at a few

inches of water column. The digester should be designed for

the maximum pressure and vacuum.

CHAPTER 3STABILITY CONSIDERATIONS

3.1Flood considerations

3.1.1Determination of flood designSpecial considerations

are required for the design and construction of reinforcedconcrete structures that are subjected to forces caused by

external flooding. Such flooding could be due to surface

water, rising ground water, or a combination of both.

Environmental structures, particularly wastewater treatment

plants, are frequently sited in areas subject to stream

flooding and high ground water tables, where hydrostatic

uplift pressures can significantly reduce the overall stability

of the structure. Flood hazard maps, as produced by the

Federal Emergency Management Agency (FEMA), may be

available for sites in the vicinity of streams and rivers.

Information and FEMA flood maps are available on the web

at http://www.hazardmaps.gov/atlas.php. Most projects are

designed for a 100-year flood event, unless higher flood

levels such as the probable maximum flood, are appropriate

for critical structures. The term probable maximum flood

refers to the flood that would result from the critical combination

of precipitation, ground saturation, and runoff factors

considered reasonably possible in a particular drainage

basin. Because of increased costs involved in protecting

against the probable maximum flood and its extremely small

chance of occurrence, its application is normally limited to

design of spillways for dams, the sudden failure of which

would result in extraordinary hazards to human life or in

disastrous property damage.10



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Where flood maps are not available, meteorological

records may be available for evaluation of elevations and

frequency of flooding for a given location. A qualified

individual should review the available records to determine

the project design flood.11 The reliability of flood-frequency

estimates is limited by the length and quality of hydrologic

records available for the project drainage basin, the records

accuracy with respect to long-period characteristics, the

probability of changes in factors that influence flood

characteristics, the adequacy of analytical methods used,

and other considerations. Experience and analysis have

shown that estimates of flood data are usually subject to a

relatively large margin of error and that extrapolations to

events substantially less frequent are of questionable value in

selecting design-flood criteria for a particular project. These

factors should be considered in the selection of an appropriate

project design flood and applicable factors of safety.

3.1.2Flotation safety factorsThe safety factor against

flotation is usually computed as the total dead weight of the

structure divided by the total hydrostatic uplift force. Weight of

soil directly above structural components may also be consideredin the dead weight. In some cases, frictional resistance due to soil

embedment or a soil failure wedge may be considered to increase

resistance against flotation. The hydrostatic uplift force may be

determined either by calculating the displaced volume and

multiplying that volume by the density of water or by calculating

uplift forces due to hydrostatic pressures at the base of the

structure. Eccentricities between the centroid of the dead weight

and the centroid of the applied uplift should be considered.

The flotation safety factor should reflect the risk associated

with the hydrostatic loading condition. Commonly used

safety factors are 1.10 for worst-case conditions, such as

flood to the top of structure and using dead weight resistance

only, or 1.25 for well-defined design flood conditions (such asfrom a FEMA flood map) below the top of structure. A

minimum safety factor of 1.25 is also recommended

where high ground water conditions exist. Where

maximum ground water or flood levels are not well

defined, or where soil friction is included in flotation

resistance, higher safety factors should be used. In any

case, the maximum design condition need not exceed the

condition of water to the top of structure, as the structure

would either flood or become submerged under higher

flood levels. Factors of safety against flotation apply to

overall structure stability; individual structure components

should be designed in accordance with ACI 350 for the

design flood and ground water conditions.

3.1.3 Design for flotation resistanceStructures that

extend below the design flood level or the maximum ground

water level should resist flotation. If the dead weight of the

structure does not provide an adequate factor of safety

against flotation, then the following options should be

considered, individually or in combination:

Increase the base slab thickness, roof slab thickness,

wall thicknesses, or soil cover to increase flotation

resistance. When increasing the base slab thickness, be

sure to account for increased uplift forces if the structure

depth is increased.

Extend the base slab beyond the walls of the structure

to engage soil overburden loads to increase flotation

resistance. Hydrostatic uplift at such slab extensions

should be accounted for in the analysis.

Depending on foundation conditions, tension piles,

drilled piers, or drilled soil or rock anchors may be used

resist uplift pressures. Overall flotation stability,

including the buoyant weight of the engaged soil or rockmass, should be considered in addition to design of

individual piles or anchors. Also note that where softer

soils overlie stiffer soils or rock, compressive loads from

the structure and settlement should also be considered

when using these foundation systems.

Where ground water can be drained away from the

structure by gravity, a well-designed drainage system may

be relied on to lower ground water. Perforated pipes

embedded in free-draining material and provided with pipe

cleanouts provide improved reliability. It may be difficult

or impractical to provide reliable ground drainage systems

that are subject to flooding conditions.

The above methods of improving flotation resistance areconsidered to be passive systems, as they do not rely on

human intervention or on a mechanical system becoming or

remaining active. Where such passive systems are not practical,

the following active systems may be considered:

In some cases, uplift pressures can be reduced by using

foundation drainage systems in combination with pumps.

Where drainage systems rely on pumping to maintain low

ground water conditions at the location of a structure,

backup pumps and emergency power should be provided

to ensure reliability, or there should be specific plant-

operating procedures to address precautions to be taken

before dewatering a basin or tank.

In some cases, uplift pressures can be limited by the useof pressure-relief systems in the base slab or sidewalls.

Flap valves in sidewalls are generally considered to

be more reliable than pop-up valves in base slabs.

Pressure-relief systems can become clogged and non-

functional, especially when used where sludge may collect

in the vicinity of the relief valve. Also, open valves may

interfere with mechanisms such as clarifier scrapers.

Where relief valves are used, ground water should be

assumed to be at least 1 ft (300 mm) above the elevation of

the relief valve due to the pressure required to open the

valve and the possible head buildup in the surrounding

soil. The engineer should consider a drainage layer

below the entire base slab to be hydraulically connected

to the relief valves. This will reduce the possibility

of pressure buildup between drained areas and will

improve reliability. The rapid drainage of a tank or

basin might not allow sufficient time for relief valves to

lower ground water. Relief valves, which permit external

water to enter and mix with the contained water, are not

permitted to be used in potable water applications, such

as in finished water tanks.

Slab blowout panels may be provided where allowance

can be made for repair time, if needed. The blowout

panel should be designed to fail and allow water intrusion



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before damage occurs to the remainder of the structure.

Significant downtime may result if failure of the blowout

panel occurs. Thus, such blowout panels are often used

as a backup to other active systems.

When none of these methods are practical, and when

approved by the owner, an alarm system may be

considered. The alarm system should alert the operator

when a dangerous ground water elevation is reached so thatthe operator can start filling the containment structure to

balance the uplift. There should be at least two independent

alarm systems that are tested at regular intervals. Plant-

operating manuals should include procedures to be

followed when alarms indicate required action.

Higher safety factors are recommended for active systems

than for passive systems because they may not be as reliable

in an emergency event. Monitoring wells may also be advisable

for monitoring ground water levels when using the above

active systems for flotation resistance.

3.2Sliding and overturning considerationsSliding and overturning can occur to environmental structures

and to individual components of environmental structures due

to unbalanced soil conditions, unbalanced liquid levels, or

wind or earthquake effects. The required factor of safety

against sliding may be adjusted based on risk and probability of

loading conditions, similarly to adjustments in load

factors for such conditions. The sliding safety factor also

accounts for the uncertainty that exists in determining in-place

soil properties. The bearing capacity and minimum base

area in compression, which also provide overturning

resistance, may also be adjusted according to the design

loading condition.

The minimum recommended safety factor against sliding,the minimum recommended base area to remain in

compression, and the recommended bearing capacity safety

factor are included in Table 3.1 for various loading conditions.

The usual condition includes loading conditions that are

expected to occur during normal operation of the plant. The

earthquake condition refers to load conditions that include

seismic forces. The unusual condition includes temporary

construction conditions and worst-case fluid levels. Special

consideration should be given to stability at corners of liquid-

containment structures and to effects of joint discontinuities,

such as at expansion joints, on structural stability.

The sliding safety factor is determined by dividing the

calculated sliding resistance along an assumed sliding plane

by the applied shear forces acting along that plane. Sliding

resistance should be based on soil friction, cohesion, or both,

as applicable. Applied shear forces may be reduced by

resisting earth or fluid pressures, as applicable. Whendesigning a structure to satisfy a given factor of safety

against sliding, the engineer should confirm that a realistic

coefficient of friction is being used to calculate the factor of

safety provided. The use of an excessively low coefficient of

friction in design may result in a greater degree of conservatism

than is intended. If a key is used to prevent sliding, the earth

pressure on both sides of the key should be considered in the

analysis. If passive earth pressure is being generated on one

side of the key, generally, active pressure is occurring on the

other side of the key. The resistance to sliding provided by

the passive earth pressure is reduced somewhat by the

opposing active earth pressure. Where hydrostatic uplift

exists for a given loading condition, the uplift forceshould be included in the total forces on the sliding plane

(that is, the sliding resistance may need to be reduced due

to the buoyant effect.)

When the resultant falls within the middle 1/3 of the base,

assuming a rigid rectangular foundation, the maximum and

minimum pressure may generally be calculated using the

following equations for a unit width, uniform strip.5

(3-1)

(3-2)

where

B = width of the base;

e = eccentricity ofP relative to the centerline of the base;

P = total applied force normal to the foundation plane;

qmax = maximum foundation bearing pressure; and

qmin = minimum foundation bearing pressure.

When the resultant falls outside of the middle 1/3 of the

base, assuming a rigid rectangular foundation, the maximum

base pressure may generally be calculated using the

following equation for a unit width, uniform strip12

(3-3)

The base width remaining in compression may generally

be calculated using the following equation

qma xP

B---

1 6eB------+

=

qmi nP

B---

1 6eB------

=

q

4P

3-------

B 2e---------------=

Table 3.1*Safety factors for stability5

Loading condition

Minimumsafety factor

against sliding

Minimum basearea in

compression

Minimumfoundation bearing

capacity safety factor

Usual 1.5 100% 3.0

Unusual 1.33 75% 2.0

Earthquake 1.10Resultant

within the base 1.0

*Table is not intended to apply to retaining structures that rely on anchorage devices, suchas rock or soil anchors, for stability. Loads used to calculate safety factors shouldbe service loads.Bearing-capacity safety factors may be adjusted based on recommendations ofgeotechnical engineer considering site-specific geotechnical conditions.Low safety factor was established based on short-term nature of load, the ability ofsoil to resist higher short-term loads, and the rarity of sliding and overturning failuresin earthquakes.



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(3-4)

whereBc = base width remaining in compression.

CHAPTER 4SPECIAL DESIGN CONDITIONS4.1Load combinations

4.1.1 Loading combinations for wallsWalls of liquid-containing structures are commonly subjected both to liquid

pressure from the inside and earth pressure from the

outside. Such walls and their supports should be designed

for each of these loads acting independently. External

earth pressure generally should not be relied on to resist

liquid pressure because:

Wall backfill may not be in place when the tank is filled

initially for leakage testing;

Earth pressures may be removed due to excavating for

an addition, modification, or repair adjacent to the

structure; and

The resulting change in earth pressures due to the

outward wall movements are difficult to predict.For multicell fluid-containment structures, the effects of

combinations of empty and full cells should be considered in

the design.

4.1.2Tension loadsContinuous walls and slabs between

ends of rectangular liquid-containing structures should be

designed to transfer tension forces between endwalls to

balance the lateral forces due to the contained liquid. This

direct tension force should be considered in the reinforcement

design at corners, wall-to-slab connections, and along the

entire length of the wall or slab. Where the cross section is

fully in tension, the required tensile reinforcement should be

distributed proportionately to each face. If significant flexural

loads are present, the required area of tension reinforcement

may either be conservatively added directly to the required

area of flexural tension reinforcement, or the combined

flexure and tension may be evaluated in accordance with

ACI 340R (SP-17).

4.2Expansion and contraction conditionsRoofs over environmental engineering concrete structures

often cover large areas and they may be exposed to significant

thermal movements. Stresses can occur in the walls and roof

due to difference in thermal movement between the walls and

roof. The stresses can be reduced by providing movement joints

between the walls and the roof and within the wall and roof

elements. Low-friction bearings or suitable flexible bearing

pads should be incorporated into the design of the movement

joints to prevent spalling and other damage due to direct tension.

Stresses can also occur in the walls and roof due to the

difference in temperature and moisture through the thickness

of the wall and roof section. These stresses should be considered

in the design where applicable.

Large-diameter tanks expand and contract appreciably due

to thermal changes, shrinkage, creep, and elastic deformations

as they are filled and drained. The connection between the

wall and footing of such tanks should be detailed to permit

these movements to occur freely or design them strong

Bc3 B 2e( )

2-----------------------=

enough to resist the movements without cracking. Similarly,

the roof-to-wall connection should account for such movements.

Refer toChapter 5 for additional discussion of design of

joints in concrete structures.

4.3Foundation conditionsFoundation design is critical to control cracking and

maintenance of liquid tightness in liquid-containingstructures. Differential movements can cause cracking in

structures, failure of joints, failure of rigid pipe connections,

or damage to operating equipment. Thus, variations in

foundation stiffness should be avoided; for example,

soil-supported elements should not be combined with

elements supported on deep piles or drilled piers unless

differential vertical movements are taken into consideration.

Similarly, where a portion of a structure is much shallower

than an adjacent portion of the structure, the shallower

portion may be prone to differential settlement due to the

relative depth of fill materials below it. In some cases, the

shallower portion of the structure may need to be supported

on a deep foundation.Typically, sliding resistance between the bottom of the

structure and the soil is neglected when the structure is

supported on piles or drilled piers.

4.4Design and detailing considerationsProper consolidation of concrete is essential in a liquid-

tight structure. Thin, cast-in-place, reinforced concrete walls

make placement and consolidation of concrete more

difficult. The minimum thickness for walls that are greater

than or equal to 10 ft (3 m) in height is 12 in. (300 mm)

(ACI 350-01, Section 14.6.2). For lower walls, 10 in.

(250 mm) is the practical minimum thickness for walls with

reinforcement in each face. If 8 in. (200 mm) walls areused, a single mat of reinforcement placed 2 in. (50 mm)

from one face is preferred to allow space for placing and

consolidating the concrete. Greater thicknesses are desirable

where waterstops are used, due to the limited space available

for both reinforcement and waterstop placement with

adequate concrete cover. For walls greater than 10 ft (3 m)

in height, a reasonable rule of thumb is to use a minimum

wall thickness equal to 1/12 the wall height for cantilever

walls and 1/16 the height for propped cantilever walls.

The minimum thickness of footings and mat foundations

should generally be 12 in. (300 mm).

Liquid-containment walls may be designed as cantilever

walls, propped cantilever walls, pinned endwalls, or two-

way plates with various edge conditions, depending on

support conditions. For propped cantilever walls and two-

way plate walls, the assumption of full fixity at the base may

not be entirely accurate, especially if supported on soil. A

fixed-base assumption is normally conservative for design of

the reinforcement at the base of the wall, but it may not be

conservative for midspan reinforcement. Thus, consideration

should be given to assuming partial fixity at the base. A

conservative approach is to bracket the design by

detailing wall reinforcement for both the fixed-base and

pinned-base design conditions.



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Except near corners and intersecting walls, long walls may

be designed as one-way vertical spans. Near corners and

intersecting walls, two-way plate action should be considered. A

reasonable approximation of corner moments is to use two-

way plate design tables based on a wall with the maximum

tabulated aspect ratio.

Where two-way plate tables are used for wall design, the

actual fluid depth may be somewhat less than the full wallheight, as shown in the design tables. An approximate design

method is to calculate the total lateral force on a unit length

of wall due to the actual fluid depth and then redistribute this

total lateral force over the full height of the wall.

The minimum required area of flexural tension reinforcement

for structural slabs and footings of uniform thickness can be

reduced to that permitted for minimum shrinkage and

temperature reinforcement per ACI 350. Where one-way

slabs and walls are uniformly loaded by liquid or earth loads

and are intended to be liquid-tight, consideration should be

given to the use of the minimum recommended flexural

reinforcement as required for beams where an overload

condition could result in a sudden or brittle failure.Reinforcement in each face of structural members should

be extended for the full length of the member for improved

crack control and structural ductility. For crack control,

smaller bars at closer spacing are preferred over larger bars

at a wider spacing.

4.5Vibration conditions4.5.1GeneralMost types of process equipment associated

with treatment plants, such as clarifiers, scrapers, and paddle

mixers, are relatively slow moving and will not cause structural

vibrations. For such equipment, a separate design for dynamic

loading is unnecessary. Other commonly used equipment, such

as centrifugal pumps, fans, blowers, compressors, centrifuges,and generator engines, however, can have much higher

operational frequencies and will require special consideration in

the design of their support structures and foundations. Usually,

the value of such equipment and its reliability is much greater

than the cost of the foundation, so that it is imprudent to

economize on foundation costs and run the risk of shortened

machine life, increased maintenance costs, and breakdowns.

In treatment plants, machines that most often cause vibration

problems are large forced-draft fans (blowers), as used for

aeration tanks, and centrifuges, as used for dewatering of

sludge. These are very sensitive machines and require carefully

designed foundations to prevent resonant vibration. Chemical

mixers may also produce of significant dynamic loads that

should be considered in the design of their supports.

4.5.2 Design for vibrationThe key to successful

dynamic design is to ensure that the natural frequency of the

equipment support structure is significantly different from

the frequency of the disturbing force caused by the equipment.

If the two frequencies approach each other, resonant vibrations

will occur in the support structure. Resonant vibrations in the

support structure that result in large amplitude oscillations

can cause damage to the structure, to the equipment, or both.

To minimize resonant vibrations, the ratio of the natural

frequency of the structure to the frequency of the disturbing

force should be outside of the range of 0.5 to 1.5. It is difficult to

obtain an accurate determination of the natural frequency of

concrete members due to variations in concrete strength,

modulus of elasticity, and cracked or uncracked sections.

Theoretically, the structure can be designed to have a

natural frequency that is lower than the operating frequency

of the equipment. This practice is commonly referred to as

low tuning and is intended to avoid resonant vibration. Thismethod is generally not recommended, but sometimes cannot

be avoided. Low-tuned structures tend to be relatively flexible

and have higher deflections. Low tuning is most practical for

equipment that has an operating frequency that is much

higher than the natural frequency that can be practically

achieved for the supporting structure. A disadvantage of low

tuning is that the machine would pass through the resonant

frequency of the supporting structure at startup and shut-

down. If the resonant condition is transient, it is unlikely that

it would cause damage to the machine, but it is generally

preferable to keep operational deflections low. Many types

of equipment come up to full operating speed in a few

seconds. For this equipment, the short period when a resonantcondition exists would not be long enough to allow excessive

amplitude to build. The resonant condition can be more of

a concern for equipment that takes a long time to reach full

operating speed. Of special concern is equipment where the

operating speed can be adjusted over a range that includes the

resonant frequency of the supporting structure. If the equipment

was set to operate at the resonant frequency of the supporting

structure, excessive amplitude may occur, possibly causing

damage to the supporting structure and the equipment.

Furthermore, if the supporting structure is stiffer than

estimated, its natural frequency may be higher than calculated.

Some causes of the supporting structure being stiffer than

calculated would include the concrete being stronger than thedesign strength, a cracked section being assumed for calculations

and the concrete cracks less than assumed, construction

tolerances resulting in larger members than used in design, and

the length of members used in calculations not taking into

account the member thickness at joints. Simplified analysis

methods that overestimate structure deflections may also

result in the structures actual natural frequency being higher

than calculated. Therefore, it is preferred to use a stiff structural

support system with a natural frequency at least 1-1/2 times

the equipment frequency (also known as high tuning). With

high tuning, any underestimation of the support structure

natural frequency will not create a risk of resonant vibration.

If the equipment is not supported directly on a solid

foundation but is supported on columns and beams, the

natural frequency of the support members is of primary

importance. The supporting members should have both

sufficient stiffness and strength to eliminate the risk of

resonant vibrations. It may be desirable to use vibration

isolators, especially on elevated structures, to reduce the

magnitude of vibrations transferred to the structure. This

should not be used as an alternate to considering dynamic

effects because all isolators transmit some vibration. The

amount of equipment vibration transmitted through the

vibration isolators will vary with the operating frequency of



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the equipment and the design of the vibration isolators.

Detailed information on transmissibility can be obtained

from the isolator vendor.

The natural frequencies of a structure should be calculated for

the vertical direction and for the two principal horizontal

directions. In some cases, torsional frequency may also need to

be examined. To combine the effects of several masses, thecombined natural frequency can be estimated using the Raleigh

Method. Where masses are not closely coupled dynamically

(that is, natural frequencies are not too close to one another), the

following Southwell-Dunkerley12 formula may be used

(4-1)

where

Fn = combined natural frequency;

F1 = natural frequency due to mass 1;

F2 = natural frequency due to mass 2;

F3 = natural frequency due to mass 3; and

F4 = natural frequency due to mass 4.

Drilled-in expansion anchors that rely solely on friction

for pullout resistance should not be used to anchor vibrating

equipment, as they can work loose. Cast-in-place anchor

bolts are preferred. Alternately, epoxy-grouted anchor bolts

may be considered where drilled-in anchors are necessary.

4.5.3 Calculating natural frequencyReadily available

commercial software programs can be used to calculate the

natural frequency of structures and members of structures.

Formulas are also available in technical books on vibration

for calculating the natural frequency of beams.

A method of estimating the natural frequency of a structure is

to apply gravity to the masses being considered, with gravity

applied in the direction being considered (that is, apply

gravity horizontally instead of vertically for horizontal

natural frequency). Then calculate the deflection due to this

loading assumption and calculate the fundamental natural

frequency (due to the first mode of vibration) from the

following formula for a single degree of freedom system

(4-2)

where

D = calculated deflection;

Fn = fundamental natural frequency; and

g = acceleration due to gravity.

This formula is derived from the general equation for natural

frequency of a single degree of freedom system as follows

1

F2

n

--------1

F2

1

--------1

F2

2

--------1

F2

3

--------1

F2

4

-------- ...+ + + +=

Fn1

2------

g

D----=

(4-3)

where

Fn = fundamental natural frequency;

k = W/D = spring constant (force per unit deflection);

M = W/g = mass; and

W = weight of considered mass.

The masses multiplied by the acceleration due to gravity

(equivalent to the weight of the masses under consideration)

are applied to obtain the effective spring constant of the

structure, which is directly related to the stiffness of the

structure as calculated by its deflection. Thus, the deflection

calculation for determining natural frequency should not

include any live loads or dynamic operational loads. The

deflection calculations should only include the weights

associated with the masses that contribute to the

frequency of the system (for example, the equipment

mass, the mass of any contained fluids, and the

supporting structure mass).

Natural frequency of beams in cycles per second can be

calculated using the expressions given in Table 4.1. These

equations provide a simple method of calculating the

approximate natural frequency of a beam from its static

deflection;D is the immediate elastic deflection at the noted

location due to the uniform or concentrated weight applied in the

direction under consideration to a beam with the noted end

Fn1

2------

k

M-----=

Table 4.1Natural frequency of beams

End conditionEnd 1-End 2 Load

Position ofdeflectionD

Naturalfrequency

(cycles per s)

Fixed-Free Uniform End 2

Pin-Pin orFixed- Fixed

Uniform Midspan

Fixed-Fixed orFixed-Free or

Pin-Pin

Concentrated(any position)

Under load

Notes:D = deflection in inches (mm); g = 386 in./s2 (9804 mm/s2).Multiply natural frequency by 60 to calculate RPM.

0.20g

D----

0.18g

D----

0.16g

D----

Table 4.2Recommended maximum staticdeflections of beams supportingvibratory equipment

Operating frequencyof equipment,cycles per min

Minimum naturalfrequency of

structure, cyclesper min

Maximum static deflectiondue to dead load and

equipment load,in. (mm)

400 600 0.10 (2.5)

600 900 0.044 (1.1)

800 1200 0.025 (0.64)

1000 1500 0.016 (0.41)

1200 1800 0.011 (0.28)

2000 3000 0.004 (0.10)

2400 3600 0.0027 (0.069)



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conditions. Instead of a comprehensive deflection analysis, D

can be estimated using the methods outlined in ACI 318.

Table 4.2 shows the minimum recommended natural

frequency of the structure to be at least 1-1/2 times the operating

speed of the equipment. Table 4.2 also shows the maximum

static deflection permissible for the supporting structure, as

calculated using the equation for a concentrated mass on a beam.

The effect of the mass of the beam can be approximated as aconcentrated load at the center of the beam, or a more refined

analysis accounting for uniform loads can be developed. This

approximation will result in a larger deflection and in a lower

calculated natural frequency. The resulting natural frequency

should be evaluated in comparison with the equipment operating

frequency and checked for susceptibility for resonance. The

small deflections (or stiffer structures) required for high-

frequency equipment (that is, greater than 1200 cycles per

minute) may not be practical to achieve, and low tuning may be

required. Motors used to drive pumps and other equipment often

operate at high rotational speeds, but electrical motors are

generally well balanced and do not tend to cause significant

vibrational problems. Consequently, motor supports aregenerally not designed for vibrational effects due to the usually

high motor speed, but such supports should be relatively stiff to

avoid vibrational problems. Vibration isolators may be desirable

at the motor support points.

4.5.4Foundations at gradeWhere dynamic equipment

loads are to be supported using spread foundations, the safe

design bearing load is often assumed as 1/2 of the allowable

foundation bearing load for a statically loaded foundation at

the same location.13,14 Some sources recommend minimum

foundation-to-equipment mass ratios, generally in the range

of 2 to 5.9 Adherence to this rule has not always proven

satisfactory,15 and to ensure that critical equipment will not

be in resonance with the foundation, the natural frequency of thefoundation system should be calculated. For natural

frequency calculations, follow the methods recommended

in Reference 13. For pile-supported foundations, follow the

recommendations found in References 16 through 18. Often

the equipment foundation is intentionally isolated from the

floors or other parts of the structure to minimize the transmission

of vibrations to other parts of the structure.

Block foundations should typically be at least 24 in. (600 mm)

in thickness and have a width greater than the vertical distance

from the top of foundation to the center of gravity of the

equipment. Preferably, the center of gravity of the foundation in

plan should match the center of gravity of the equipment.12

More detailed information on dynamic analysis methods

and the dynamic design of foundations can be found in

References 19 through 23. Additional information may be

available for the design of equipment foundations from ACI

Committee 351, Foundations for Equipment and Machinery.

4.6Hazardous design conditionsSuperstructures of environmental engineering concrete

structures are frequently similar to conventional structures.

In some facilities, the gases generated within the structure,

adjacent to it, or below it may be toxic or may present an

explosion hazard. Where buildings or equipment rooms are

located over or adjacent to tanks or digesters, the tanks may

require gas-proofing by means of liners, installation of gas-

detection equipment, or both.

At least one access point should be provided into each

enclosed cell for inspection and maintenance. Where practical,

two access points should be provided at opposite ends of

enclosed structures for improved safety.

Confined space entry (CSE) requirements, as defined byOSHA, should be considered in the design of all below-grade or

unventilated structures. Provisions for adequate ventilation

and safe exit passage may eliminate CSE limitations for

operating personnel.

4.7Corrosive conditionsSpecial consideration should also be given to the possible

hazardous and corrosive effects of gases such as chlorine,

oxygen, ozone, hydrogen sulfide, and methane gases in

closed tanks. Corrosion-resistant linings may be required

to protect the concrete, especially where hydrogen

sulfide gases occur above the water surface in closed

structures. Corrosive effects on metal embedments are

also very important. Furthermore, consideration should

be given to corrosive effects of stored or applied chemi-

cals in the treatment process. Where hazardous chemicals

are stored, containment basins should be provided to contain the

chemicals in case of leakage. Corrosion-resistant linings in the

containment basins may be necessary to protect the concrete

in the event of a spill or leakage. Refer to ACI 350-01 for

a listing of chemicals commonly associated with environmental

structures and their relative effects on concrete.

4.8Construction conditions

Low-permeability concrete is obtained by using a water-cementitious material ratio (w/cm) as low as possible and

consistent with satisfactory workability and good consolidation.

The use of a water-reducing admixture will generally permit the

use of a lower w/cm. Air entrainment increases workability

and reduces segregation and bleeding, thus, the use of an air-

entraining admixture generally permits the use of a lower w/cm.

Pozzolans also improve workability and reduce permeability.

Permeability is reduced with extended moist curing and with

the use of smooth forms or troweling. The filling and

patching of tie holes is also essential to long-term durability.

Form ties with waterstop collars should be used in walls

intended to be liquid-tight.

Liquid-containment structures are generally tested for

liquid-tightness before backfilling around the structure. Such

testing should be performed in accordance with the

recommendations of ACI 350.1.

The contractor is normally responsible for construction

loading conditions. The engineer, however, should anticipate

interim loading conditions, design for such conditions, and

note permitted interim conditions on the drawings where

such designs will simplify construction and reduce costs. For

example, a wall may be designed to resist backfill loads

before the top slab is in place, where it will simplify

construction. This condition may be especially important if



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an upper slab-on-ground cannot be constructed until backfill

is placed against a lower foundation wall.

Where the dewatering of excavations is required due to the

presence of ground water, the engineer should identify at

what stage of construction dewatering can be terminated

based on the strength and flotation resistance of the structure.

CHAPTER 5JOINTS IN CONCRETE5.1GeneralThe engineer should consider the structural effects of joint

locations and details on the design. Generally, the engineer

should show all required joints on the construction documents.

Any joints proposed to be added or deleted by the contractor

should be reviewed by the engineer after consideration of

effects on the structures design and crack control.

Movement joints permit dimensional changes in concrete

due to load, thermal effects, drying shrinkage, or differential

foundation movements; serve to separate or isolate areas or

members that could be affected by such dimensional

changes; and allow relative in-plane movements or

displacements due to such dimensional changes. Dowelsare often used in movement joints to resist out-of-plane

movements. The design should provide for slippage of the

dowel on one side of the joint. Keyways may be used in

nonmovement joints but should be used with caution in

movement joints due to the potential for a crack to occur

across the keyway and around the outside of the water-

stop. Figure 5.1 illustrates some of the most common

types of movement joints.

All joints should be considered as potential sources of

leakage. Leakage tends to occur more frequently in movement

joints, with expansion joints being the most likely to leak.

While closer spacing of movement joints may reduce

cracking between joints, the additional joints may increasethe potential for overall basin leakage. Thus, increased joint

spacing, in conjunction with increased reinforcement to

control crack widths, may reduce total basin leakage.

Effects of movement joints on load transfer should be

considered in design. For example, at joints in which

reinforcement is interrupted, the transfer of tension across

the joint is also interrupted. Movement joints may also affect

continuity of shearwalls or diaphragms.

Where joint materials will be in contact with water being

treated for use as potable water, the materials must be

nontoxic, generally after 30 days of curing.

5.2Construction jointsConstruction joints are generally located at natural breaks in

concrete placements, such as between wall and slab placements,

and at intervals to limit length and volume of concrete

placements. Movement joints also function as effective

construction joints, that is, breaks in concrete placements. Unless

otherwise noted or shown, construction joints are typically

designed for full transfer of stresses across the joint. Keyways or

roughened surfaces with shear-friction reinforcement are

typically used for shear transfer at construction joints.

Cracking can occur in long wall or slab placements due to

the effects of thermal movements and drying shrinkage in

combination with movement restraints on the element. The

maximum length of wall placed at one time for conventionally

reinforced straight walls should usually not exceed 60 ft

(18 m), with 30 to 50 ft (9 to 15 m) being more common.2 At

vertical construction joints, 48 h should be allowed between

placement of adjacent wall sections. Extra horizontal

reinforcement may be desirable near the base of the wall to

assist in control of cracks near the base.Large circular basins typically use only construction joints

and no movement joints. Typically, a center circular

concrete placement is made, with the remainder of the slabs

and walls placed in equal segments.

5.3Movement joints

5.3.1 Contraction jointsContraction joints are often

used to dissipate shrinkage stresses and to control cracking.

Where used, contraction joints should be located at intervals

not exceeding 30 ft (9 m), unless additional reinforcement is

provided as required in ACI 350-01, Section 7.12. Shrinkage

will occur regardless of the amount of reinforcement

provided; however, the increased reinforcement tends todistribute cracks and limit crack widths. Because cracks

occur at weak points, the intent of contraction joints is to

concentrate full-depth cracks at specific locations where

measures can be taken to protect against leakage.

Two types of contraction joints, known as full and partial

contraction joints, are in common use. In full contraction

joints, all reinforcement should be terminated 2 in. (50 mm)

clear of the joint. In partial contraction joints, at least 50% of

the reinforcement should be terminated 2 in. (50 mm) clear

of the joint, with the remainder being continuous through the

joint. The surface of the joint should be treated to prevent

bonding with the adjacent concrete placement where freshconcrete is placed against hardened concrete. Full contraction

joints provide less restraint against shrinkage at the joint;

however, partial contraction joints provide some shear

transfer and limit differential movements across the joint due

to partial continuity of the reinforcement. Because the use of

a partial contraction joint creates a weak plane, consideration

should be given to seismic performance of these joints (for

Fig. 5.1Movement joints.



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example, effects due to high, localized energy dissipation).

Partial contraction joints are typically spaced at about 2/3 the

spacing used for full contraction joints. Even though

partially reinforced, such joints should be assumed to

open when considering the moment, shear, and diaphragm

shear capacity at such joints.

5.3.2Expansion joints and isolation jointsExpansion joints

allow for thermal expansion and act as effective contraction joints. Expansion joints tend to have the most problems with

long-term leakage in liquid-containment structures, so their

usage should be limited. Expansion joints are generally used in

very long structures (typically over 150 ft [45 m] in length) or

where abrupt changes in configuration or support occur.

All expansion joints should include some type of

preformed compressible filler and a joint sealant at the liquid

face. If the structure must be liquid-tight, a suitable waterstop

should be included to act as the primary barrier against leakage.

The waterstop, preformed filler, and joint sealant should be

selected to allow for the anticipated movement and should be

suitable for the service environment. Refer to ACI 504R for

recommended joint design and materials of construction.

Isolation joints serve to separate portions of a structure,

such as between vibrating equipment foundations and building

foundations or between column foundations and floating slabs.

Such joints should include some type of preformed

compressible filler and a joint sealant at the exposed face to

keep dirt, debris, and water from entering the joint, as well as a

waterstop, where required, for liquid tightness.

5.4WaterstopsWaterstops are required in all types of joints where liquid

tightness is required. Rubber waterstops permit the greatest joint

movement and last indefinitely when placed in a dark and humidenvironment that is not corrosive to rubber. Polyvinyl chloride

(PVC) waterstops permit somewhat less movement than rubber

but are less sensitive to light exposure during construction and to

drying. Another advantage of PVC waterstops is the simplicity

of splicing by applying heat. The minimum sizes of either type

of flexible waterstop commonly used in environmental

structures is 3/8 x 9 in. (10 x 230 mm) for expansion joints and

1/4 x 6 in. (6 x 150 mm) for construction or contraction joints.

Minimum 3/8 x 6 in. (10 x 150 mm) waterstops should be

used at wall base joints where possible to avoid folding

under wet concrete placement. Copper waterstops have

also been used effectively. Stainless steel waterstops are

typically used in ozone process environments, due to the

damaging effect of ozone on other materials used for

waterstops. Refer to ACI 504R for additional discussion

of waterstop materials.

Rigid (metal) waterstops should not be used in movement

joints. Rubber or PVC waterstops with a center bulbs are

common in movement joints. Concrete cover at flat waterstops

should be equal to or greater than 1/2 the width of the waterstop.

Adequate support and careful concrete placement should be

provided to ensure proper position and embedment of the water-

stop in the concrete. For vertical waterstops, the use of tie wires

tied to the reinforcement to anchor the waterstop in position

should be considered. For flexible horizontal waterstops, the

following construction sequence should be considered:

a. Fold the waterstop upward along its entire length and

hold in position;

b. Place and consolidate concrete up to the elevation of the

waterstop;

c. Reposition the waterstop into the top of the fresh

concrete; andd. Complete concrete placement and consolidation around

the remainder of the waterstop.

A number of alternative waterstop types have become

available, such as premolded adhesive waterstops;

premolded, expansive, water-reactive waterstops; and injection

tube waterstops that are injected with water-reactive chemical

grout. Some waterstops are available that combine more than

one of the above types. The effectiveness and applicability of

a particular waterstop system to any given situation should

be evaluated by the engineer. Some of these alternative

waterstops can provide solutions where new construction is

to be tied to existing construction and should be liquid-tight

or where backup systems to ensure liquid-tightness arerequired. Waterstops that expand and contract with varying

exposure to moisture may leak initially until the reaction

with moisture takes full effect. Such waterstops may be less

desirable where frequent wetting-and-drying cycles may

occur or where water is not in contact with the joint during

normal operations. Also, such waterstops may not be effective

for containment of liquids other than water, and most of these

alternative waterstops are not recommended for movement

joints. Where such alternative waterstops intersect rubber or

PVC waterstops, the method of connecting for continuity of

liquid tightness should be specified.

5.5Joint fillersPreformed joint filler serves dual tasks: it serves as a form for

Documents

Design Considerations for Environmental